The Fischer-Tropsch process can be used for the conversion of hydrocarbonaceous feed stocks into liquid and/or solid hydrocarbons. The feed stock (e.g. natural gas, associated gas and/or coal-bed methane, residual oil fractions, coal) is converted in a first step into a mixture of hydrogen and carbon monoxide (this mixture is often referred to as synthesis gas or syngas). The synthesis gas is then converted in a second step over a suitable catalyst at elevated temperature and pressure into predominantly paraffinic compounds ranging from methane to high molecular weight molecules comprising up to 200 carbon atoms, or, under particular circumstances, even more.
Numerous types of reactor systems have been developed for carrying out the Fischer-Tropsch reaction. For example, Fischer-Tropsch reactor systems include fixed bed reactors, especially multi tubular fixed bed reactors, fluidized bed reactors, such as entrained fluidized bed reactors and fixed fluidized bed reactors, and slurry bed reactors such as three-phase slurry bubble columns and ebullated bed reactors. The Fischer-Tropsch reaction is very exothermic and temperature sensitive with the result that careful temperature control is required to maintain optimum operation conditions and desired hydrocarbon product selectivity. Bearing in mind the very high heat of reaction which characterises the Fischer-Tropsch reaction, the heat transfer characteristics and cooling mechanisms of a reactor are very important in order to remove heat efficiently from the reactor and avoid potential temperature runaways and obtain optimal product slate. The heat transfer performance of a fixed-bed reactor operated in trickle mode is limited because of the high gas hold up (low heat capacity), relatively low mass velocity and small catalyst particle size. If one attempts, however, to improve the heat transfer by increasing the gas velocity (and subsequently the reactor temperature), a higher CO conversion could be obtained, but an excessive pressure drop across the reactor may develop, which limits commercial viability. Increasing reactor capacity by increasing gas throughput and CO conversion may also result in increasing radial temperature gradients. For thermal stability and efficient heat removal the Fischer-Tropsch fixed-bed reactor tubes should have a diameter of less than 10 cm and preferably smaller.
The desired use of high activity catalysts in Fischer-Tropsch fixed-bed reactors makes the situation even more challenging. The limited heat transfer performance makes local runaways (hotspots) possible, which may result in local deactivation of the catalyst. In order to avoid runaway reaction the maximum temperature within the reactor must be limited. Moreover, the presence of temperature gradients in the radial and axial directions means that some of the catalyst is operating at sub-optimal conditions. Commercial fixed-bed and three-phase slurry reactors typically utilize boiling water to remove the heat of reaction. In the fixed-bed design, individual reactor tubes are located within a jacket containing water/steam. The heat of reaction raises the temperature of the catalyst bed within each tube. This thermal energy is transferred to the tube wall forcing the water in the surrounded jacket to boil. In the slurry design, cooling tubes are most conveniently placed within the slurry volume and heat is transferred from the liquid continuous matrix to the tube walls. The production of steam within the tubes provides the needed cooling. The steam in turn may be used for heating purposes or to drive a steam turbine.
The presence of a flowing reactant gas in a reactor being liquid-full improves the radial bed conductivity and the wall heat transfer coefficients leading to efficient heat removal and temperature control A potential limitation of the trickle bed system (as well as any of the fixed-bed designs) is the pressure drop associated with operating at high mass velocities. The gas-filled voidage (bed porosity) in fixed-beds (typically less than 0.50) and size and shape of the catalyst particles does not permit high mass velocities without excessive pressure drops. Consequently, the conversion rate per unit reactor volume is limited by heat removal and pressure drop. Increasing catalyst particle size and higher mass flow rates improve heat transfer rates for a given pressure drop. However, the loss of catalyst selectivity and lower catalyst efficiency may make this unattractive.
Three-phase slurry bubble column reactors potentially offer advantages over the fixed-bed design in terms of heat transfer performance. Such reactors typically incorporate small catalyst particles in a liquid continuous matrix. The synthesis gas is bubbled through, maintaining suspension of the catalyst particles and providing the reactants. The motion of the continuous liquid matrix promotes heat transfer to achieve a high commercial productivity. The catalyst particles are moving within a liquid continuous phase, resulting in efficient transfer of heat generated in the catalyst particles to the cooling surfaces. The large liquid inventory in the reactor provides a high thermal inertia, which helps prevent rapid temperature increases that can lead to thermal runaway.
A disadvantage of such a system is that catalyst particles must be removed from the reaction products, as at least part of the reaction products are in the liquid phase under reactor conditions. This separation is typically carried out using an internal or external filtration system. Other issues associated with the use of suspended catalyst particles are non-uniform distribution of catalyst throughout the reactor (with knock-on effects on cooling), foam formation and catalyst attrition.